RSA is easy #2

Similar to RSA is easy #1, but this time we don’t get the public key.
However, the plaintext is much longer (> 1000 encrypted bytes). Because each byte is encrypted separately, we actually get an encryption that substitutes each character with a symbol, which is equivalent to a Substitution Cipher.
Substitution Ciphers can be broken by analyzing the language frequency distribution.
We used the following website to decrypt the flag: